U.S. Pat. No. 5,363,670 discloses a self-contained cooler/freezer apparatus for carrying items in a frozen or refrigerated environment. The apparatus comprises an insulated container which is divided into two portions. The first portion is utilized for item storage and the second portion houses a pressurized coolant compartment for storing a dry ice. The pressurized coolant compartment comprises removable insulation panel. In essence, the pressurized coolant compartment is a controllable heat sink. Within a short period of time, the dry ice starts to sublimate, thereby forming cold gaseous carbon dioxide at a high pressure. The cold gaseous carbon dioxide is circulated throughout the insulated container via a solenoid actuated gas feed valve, thereby further cooling the first portion of the insulated container. A thermostatic controller activates the gas feed valve based upon temperature readings from thermocouples located within the first portion of the insulated container. A pressure relief valve is positioned within the insulated container to prevent the pressure within the insulated container from building beyond a maximum value. The sublimation of the dry ice causes pressure that is relieved outside the apparatus.
When cold gaseous carbon dioxide formed from sublimation of the dry ice is conducted out of the apparatus, the carbon dioxide cannot be used for cooling anymore.